Regarding the "light bulb" test with the car off. I assume you mean the mechanic pulled the battery ground strap, connected the clamped end of the test light to the ground strap, then hooked up the other side to the + terminal of the battery.

If you have an alarm, the light will illuminate. Alarm systems pull enough juice to light up 12v test lights all day long. The light bulb test was ok way back when. It does not apply to modern vehicles that have clocks, alarms, etc.